The overall goal of this project is to understand and exploit the macroscopic effects of microscopic susceptibility variation in Nuclear Magnetic Resonance (NMR) imaging. Microscopic variations in local magnetic fields can produce large changes in NMR image contrast. We seek to understand quantitatively a broad range of sources of this susceptibility contrast: effects of the distribution of targettable, nano-sized superparamagnetic particles; intra- and extravascular effects of deoxyhemoglobin in cardiac vasculature; and the use of these phenomena for quantifying non- invasively the distribution of vessels size in vivo. We combine Monte Carlo random walk simulations and experimental studies in animal models in order to define and probe both our understanding of susceptibility contrast, as well as our ability to exploit this understanding for physiological measurement.